Current Transformer secondary will develop a very high voltage when secondary is open circuited ? Why??

Total power of transformer on primary side shall be equal to secondary side, In case of Open Circuit, Current will get nearly zero and Voltage will get high to keep the power constant so CT will Damage.   

Its Basic Ohm's Law,

For eg; A 400:5 CT is on a 11kV feeder having a load of 200A.

So our secondary current with CT will be 2.5A.

then, V1*I1= V2*I2

V2= (11kV *200A)/2.5A= 880 kV.

In effect the secondaary voltage will 880kV, So the CT will gets simply exploded.

Hope you are clear.

CT are used when the current through a line is too high and cant be measured directly. they are in different ratio like 100:5, 4000:5 which reduce the secondary current by 20 and 800 times respectively and can be measured at the secondary coil.if the secondary is kept open the secondary voltage will be 20*Vi and 800*Vi for 100:5 and 4000:5 respectively, which will be damage the measuring device used at secondary side of CT. if CT is used for protection it will be damage the relay.

Because in transformer power is same on primary and secondary side. When we decrease the current in secondary side then correspondingly voltage is increases to keep the power constant so to avoid this high voltage occurrence we short circuited the CT secondary,,,,

I hope you understand the answer..